Five Ways Robots Move: Which Is the Best?

Although robots still don't move with the freedom of living creatures, researchers are steering their machines toward the goal of fast, accurate, autonomous movement on two legs, and four, as well as flying, swimming and rolling.

By land, by air, by sea, robot-makers strive for fast, accurate motion.

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Five Ways Robots Move: Which Is the Best?

When it comes to robot motion, computer scientists find inspiration in everything from humans to cockroaches. And although man-made machines still don't move with the fluidity and freedom of living creatures, researchers are steering their robots toward the goal of fast, accurate, autonomous movement on two legs and four legs, as well as by flying, swimming and rolling. According to Marc Raibert, founder and president of Boston Dynamics, the Breakthrough Award–winning creators of Big Dog, the complexity of handling uneven terrain means ground robots are tougher to engineer than bots that move through the water and air. "On the ground, you're very close to terrain variation," Raibert says. "The sky and sea are smooth by comparison."

Here, then, is the PM handicapper's guide to which engineers face the biggest challenges in building traveling bots that get where they're going with speed, accuracy and stability.

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Two-Legged Robots

Last month, a group of researchers at Cornell University, led by professor and mechanical engineer Andy Ruina, set an unofficial record for long-distance robot walking. The machine, Ranger, covered 14.3 miles (65,185 steps) in 10 hours and 40 minutes before stopping to recharge—the farthest an untethered robot has ever walked without falling.

Ruina and his research team half-jokingly refer to Ranger as a "four-legged biped." It is designed to study two-legged mobility, but it is outfitted with two inner legs and two outer legs. Ruina says the machine moves "like a robot on crutches."

Despite Ranger's success, Ruina says the recent milestone is modest. "By my definition, no one has made a good bipedal robot yet," he says. To Ruina, impressive bipedal robots, or "humanoids," as some call them, must be energetically autonomous, consume the same amount of energy as a human covering the same terrain and distance, and possess stability and balance.

The Main Challenge: balancing energy consumption and stability. A ground-gobbling robot faces varied and potentially rough terrain, both of which require extra juice to keep the machine stable. The speed-accuracy tradeoff makes this tricky—to be stable, a robot needs to be accurate and quick, but quickness means high energy costs. Ranger is an example of a robot with low energy needs but subpar speed and stability.

Degree of Difficulty: Designing a two-legged robot for flat terrain, while an engineering challenge, is manageable. On sloped or uneven terrain, however, the robot has a tendency to topple.
Examples: PETMAN, a robot developed by Boston Dynamics for testing chemical protection clothing used by the U.S. Army; Asimo, the humanoid robot developed by Honda

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Four-Legged Robots

It might seem that a quadruped robot would have an easier time walking than a biped one, given that four legs provide a wider, more stable platform. However, two extra legs also mean more moving parts, which requires more energy and better coordination.

Ruina explains that slow four-legged walking is indeed easier than slow two-legged walking, but because of the speed-accuracy tradeoff, high-speed four-legged walkers face most of the same problems as the two-legged variety.

Degree of Difficulty: Four-legged robots are better than two-legged ones for sloped or uneven terrain, assuming you get the coordination between the four limbs under control.

Examples: LS3, a bigger and better version of BigDog being developed for the Defense Advanced Research Projects Agency (DARPA), developed by Boston Dynamics; TITAN XI, a gigantic four-legged robot used for construction, made by Hirose-Fukushima Robotics Lab in Japan

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Flying Robots

It's relatively easy for unmanned aerial vehicles (UAVs) to maneuver through the air, which is generally smooth, except for pockets of turbulence, and obstacle-free, aside from birds, towers and other aircraft. UAVs also benefit from advanced flight technology—the jump from remote-controlled planes to flying robots is a relatively small one. That is, unless you're an engineer in one of the labs around the world that are tackling the flapping-wing problem by designing robotic bats and insects that could one day be used for military surveillance.

Degree of Difficulty: Plane-like flight is old hat for engineers, but flapping wings are a serious challenge for roboticists to successfully implement.

Examples: Swarming Micro Air Vehicle Network (SMAVNET), a swarm of gliding robots developed at the École Polytechnique Federale de Lausanne; DARPA-funded cyborg beetles developed at the University of California, Berkeley

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Swimming Robots

Like the sky, a large body of water is a relatively homogenous and unobstructed medium, so the land-centric issues of variable energy consumption to maintain speed and balance over changing terrain do not come into play. And, as with flying robots, underwater robots benefit from existing boat technology. The Virginia Institute of Marine Science's Fetch1, for example, deploys familiar means of propulsion and control—propellers, rudders and fins.

And, as with flapping technology, the next big challenge for aquatic engineers is to design a machine that undulates like a fish to propel itself forward. Though these robotic fish would likely be used for the same applications as boat- or submarine-like robots—such as detecting water pollution—the debate continues as to whether boat propulsion or natural propulsion is more energy-efficient.

Degree of Difficulty: "Researchers have no problem designing robots that can float like submarines, but mimicking the undulating motion of an actual fish is still difficult enough to make it a rare feat: Only a handful of wriggling robots have been designed to date.

Examples: RoboTuna, developed by the Boston Engineering Corp and Franklin W. Olin College of Engineering for the Navy; SOLO-TREC, NASA's underwater robot that uses renewable energy

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Rolling Robots

Hands down, this is the simplest form of robot locomotion. "If your main goal is to make a practical machine, wheels and treads are still the way to go," Ruina says. That's because engineers have had decades of experience designing this type of robot, starting with cars, which, Ruina says, "are also robots in a way."

These bots are simple—a body, a motor and a set of wheels or treads, with control provided remotely by humans or on board with computer algorithms for autonomous movement. At this point in the advancement of robotic technology, the amount of power it takes to power wheels is a lot less than it would take for a walking robot that has the same stability. In other words, you're getting significantly more bang for your robo-energy buck.

Degree of Difficulty: Easiest all-around

Examples: The Mars Rovers—Sojourner, Spirit, and Opportunity—all roll around on six wheels. iRobot's Roomba is a two-wheeled vacuum-cleaning robot.

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